1. Field of the Invention
Embodiments of the present invention generally relate to gas phase olefin polymerization. More particularly, embodiments relate to chemical passivation and/or coating treatments for gas phase polymerization.
2. Description of the Related Art
Gas phase polymerization converts a gaseous monomer into granular solids. The gaseous monomer, and optional comonomer, is introduced under pressure into a reaction vessel, i.e. reactor, containing a catalyst and optional activator. Once polymerization begins, the monomer molecules diffuse to the growing polymer chains within the reactor. The resulting polymer is obtained as a granular solid which is fluidized within the reactor with the monomer and catalyst.
Gas phase polymerization is particularly prone to depositing solid particles on the reactor walls and other process exposed surfaces of the reactor due to static charge or electrical attraction between a metal surface and the polymer. Over time, the solids can accumulate and grow to form a solid sheet of polymer on the metal, such as a reactor wall or internal distribution plate, injection nozzle or product discharge nozzle. This phenomenon is common in the art and is known as “sheeting” when the polymer forms on a wall of the reactor, or “plugging” when the polymer forms on the distributor plate or nozzles and blocks the flow of fluid therethrough.
Polymer sheets on the walls of the reactor can grow in height and thickness to the point where the weight of the sheet overcomes any attractive forces between the sheet and the metal and falls to the bottom of the reactor. Solid polymer can also grow to an appreciable thickness to plug or block fluid flow paths on the nozzles or distribution plate. In either case, the solid polymer can plug or block monomer injection, catalyst injection, and/or product discharge. The solid polymer can also inhibit or interfere with fluidization within the reactor. As a result, the polymer product can become off spec and/or polymerization can come to a stop. To remove the solid polymer, the reactor is usually purged and shut down, which is both costly and time-consuming.
Certain pretreatment techniques have been used to prevent or control sheeting or plugging within the reactor. For example, a liquid catalyst has been sprayed onto the walls of the reactor and reacted with the monomer to produce a polymer coating or layer on the reactor wall that serves as an insulation layer to prevent product polymer growth thereon. Such technique is described in more detail in U.S. Pat. Nos. 4,532,311, 4,792,592, and 4,876,320. However, such techniques also grow solid polymer on injection nozzles and the gas distribution plate thereby inhibiting or plugging the gas flow through the reactor as discussed above. Such polymer then has to be removed before product polymerization can commence.
To remove the unwanted polymer, the reactor has to be purged before the nozzles and the gas distribution plate can be cleaned, which can consume several more days of reactor down time. However, certain areas or places on the distribution plate and nozzle are not accessible to be easily cleaned so polymer growth is left behind to accumulate and become a bigger burden over time.
There is a need, therefore, for an improved passivation and/or coating treatment for gas phase polymerization that can reduce undesired polymer growth and/or adhesion within a reactor or reactor system.